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Mrs May announced at the Conservative Party conference that she intends doing so by the end of March 2017.

The judges heard submissions that the Government lacks legal power to use the royal prerogative to notify Article 50 and begin the process of removing statutory rights granted to UK citizens under the European Communities Act 1972, which made EU law part of UK law.

Government lawyers say that if the challenge succeeds, the Government “could not give effect to the will and decision of the people, as clearly expressed in the referendum, to withdraw from the EU without further primary legislation”.

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Judges were told the Government has no legal power to trigger article 50

Because of the urgency and constitutional importance of the case, any appeal is expected to be heard by the Supreme Court, the highest court in the land, before the end of the year.

In final submissions, Government lawyers revealed that any new treaty agreement with the European Union following Brexit would “very likely” have to be ratified by Parliament.

A Government QC said the “view at the moment” was that ratification was likely if the royal prerogative was used, without prior parliamentary approval, to launch the Brexit process.